The present invention relates to the field of hydrostatic transmissions for propelling vehicles. More particularly, this invention relates to a hydraulic control cold start neutral valve for insuring that a variable displacement hydraulic unit is positively and automatically forced to a neutral position when the vehicle is shut off or the command signal is withdrawn or decreased below a predetermined threshold level. The invention further insures that the unit is in a neutral position for start up so as to reduce the amount of starter torque required.
Vehicles typically have a limited amount of starter torque available. With hydrostatic units, torque requirements increase dramatically when the oil used in the unit is cold. The oil is more viscous and difficult to move at low temperatures. Placing the yoke or swashplate of the unit near to or in a neutral position during starting of the vehicle reduces the starting torque required. Furthermore, insuring that the hydraulic unit is automatically and positively forced into a neutral condition at shut off and start up may also be desirable from a safety standpoint.
Thus, a primary objective of the present invention is the provision of an automatic hydraulic control cold start neutral valve.
Another objective of this invention is the provision of a valve that is inexpensive to construct and easy to incorporate in a variety of servo controlled hydraulic units.
Another objective of this invention is the provision of a valve that is durable and reliable in use.
These and other objectives will be apparent from the drawings, as well as from the description and claims that follow.
The present invention relates to a hydraulic neutral cold start control valve for a variable displacement hydraulic unit. The valve includes a housing with a spool bore and an intersecting signal pressure passage therein. A spool is disposed in the bore and is mechanically biased to a closed or neutral position of the valve by a pair of opposing springs. A plunger is associated with one of the springs and has a surface that usually sealingly contacts one end of the spool but extends radially therebeyond such that it is in fluid communication with the signal pressure passage.
When the signal pressure exceeds a threshold value, the pressure forces the plunger to separate from the end of the spool. Then the movement of the spool becomes proportional to the signal pressure, which is desirable for normal operation and control of the hydraulic unit. If the vehicle powering the hydraulic unit is shut off or the signal pressure is interrupted such that it drops below the threshold value, the opposing springs resume control and move the spool to a neutral position. This ensures that low torque will be required to restart the vehicle, even under cold oil conditions.
This disclosure includes two embodiments of the valve. One embodiment is configured as a three-position, four-port valve and the other embodiment is configured as a two-position, three-port valve. The cold start neutral valve of this invention is shown in one possible application with one of the spool positioning springs being carried by a feedback member that engages the yoke of a bent axis hydraulic unit.